Controls for linear actuators driven by electric motors frequently use electromechanical relays for the control of the coupling of the electric motors to a power supply. A typical control may e.g. take place between three main states, viz.: power supply interrupted with the motor short-circuited, rotation in one direction of rotation, and rotation in an opposite direction of rotation. Changes of these stationary states may e.g. be start and stop of the motor as well as reversing of the direction of rotation. It is well-known that the contact point in a relay is subjected to wear because of sparking when the current is turned on, and in particular when great currents are turned off. This stress on the contact has been found to be particularly pronounced in certain types of state changes in certain types of systems. For example, it has been found that a motor coupling, because of the inertia of the motor for a short or long period of time after a change in state, such as stopping or voltage inversion, provides a strong reactive voltage which apparently affects the performance of the relay and reduces the durability of the relay over time. Thus, it must be expected that relays for motor controls have an unreasonably short service life. Understandably, this service life is particularly difficult to handle in connection with encapsulated electronic circuits, since replacement of relays in such systems will involve relatively great costs.
In DE 3 811 799 and DE 3 516 985 a FET transistor is used in order to minimize the stress problem as described above. Before changing the state of the motor by changing the position of the relays, a FET transistor is used to deactivate the circuit. The circuit is deactivated until the motor has stopped whereupon the position of the relays and thereby the state of the motor can be changed.
This process can be quite time consuming since it is necessary to delay the change of motor state until the motor has slowed down in order to avoid unnecessary stress on the relays.
The object of the invention is to provide an electric circuit which obtains a considerably improved relay service life in operation.
In accordance with the present invention, an electric circuit is provided for an inductive load including a first relay and a second relay for connecting the inductive load with a power supply. Both relays are connected to the inductive load so as to be activated when the voltage drop across the inductive load is to be interrupted and or inverted.
A second electrically activatable relay which connects a first connecting point electrically to a second connecting point in a non-activated state and connects the first connecting point to a third connecting point in an activated state, an inductive load being coupled directly or via additional coupled electric components between the two first connecting points of the two relays so that the inductive load is connected to a power supply in given positions of the relays, said circuit being adapted to activate both relays when the voltage drop across the inductive load is to be interrupted and/or inverted, a remarkable improvement in the service life of the relays incorporated in the circuit and a high reliability in operation are achieved according to the invention.
It should be noted that an interruption of the voltage across a coupled motor is taken to mean an operation which is performed with the purpose of stopping the revolutions of the motor, while inversion of voltage supply is taken to mean an operation which is performed with the purpose of reversing the direction of rotation of coupled motors.
Irrespective of the final purpose of the operation, it will thus be appreciated that this is to be initiated by activating the relay system.
Surely, the explanation to the reason of this improvement is that the contact arms of the relays are retained better in their electrically activatable state to the connecting point in the relay, thereby avoiding strong spark discharges and strong mechanical impacts. Thus, according to the electric circuit of the invention, considerable mechanical destructive impacts on the relay arms are reduced, since these, instead of being subjected to bounce in a non-activated state, i.e. in an insufficiently purely mechanical xe2x80x9cretentionxe2x80x9d, are retained in a more controllable position in the electrically activated state.
After a given period of time, the major part of the energy generated by the electric motor, which serves as a generator for a short time after the change of state, will be dissipated in the electric circuit, i.e. in this case in the internal resistance of the motor, rather than in the mechanical contact assembly of the relays, following which the relays may be coupled, e.g. by deactivation, to the desired position, i.e. connected to the positive or negative terminal of the power supply.
Apart from the purely static considerations of the relays incorporated in the electric circuit, a greatly improved dynamic performance is achieved according to the invention, since the necessary switchings performed during the change of state take place under electrical activation of the relays rather than resilient mechanical deactivation. This means that the relay arms in the individual relays are forced against the contact point by a force from the control coil, ensuring that the bounce of the relay is minimized or avoided completely. Thus, the invention provides a well-defined switching to the contact point of the relay, as the bounce upon the impact on the contact point of the relay is minimized or avoided completely, as mentioned above. Correspondingly, deactivation of the relay coil results in contact bounce in the unenergized state of the contact assembly, caused by the oscillatory energy of the dynamic mass spring. As strong electric discharges across the contact point of the relay because of the bounce of the relay arm and a relatively high electric power given off by the motor during the braking of it are avoided, the service life of the relays used is prolonged noticeably. Pilot tests have thus shown that service life improvements of much more than a factor of 100 can be obtained.
It should moreover be noted that, in its generic embodiment, the invention may be used without supplementary current and voltage reducing measures, as the invention is precisely intended to ensure that the incorporated relays are capable of withstanding the voltages actually generated.
Thus, according to the invention, a substantially electric decoupling of inductive and electric energy is achieved altogether, so that the mechanical absorption of this electric energy in the relays may be reduced to a minimum.
The invention thus prevents the serious bounce which occurs particularly in connection with large motors which are used in linear actuators, as the repeated great sparking generated by the motor in the known systems is reduced, thereby considerably increasing the service life and reliability of the contacts. It has thus been found that, in addition to the longer service life achieved, the relays have a significantly improved service life up to break-down, if such occurs.
It should be noted in this context that the invention, in its basic form, does not take substantial measures toward reducing the combination of high voltage and current intensity generated by the motor, but more tends towards arranging the control of the relays in such a manner that these high voltages and current intensities do not cause damage to the contact points of the relay when the state is changed.
A further advantage of the invention is that the lapse of time during the change may be minimized, as the circuit of the invention does not require that the state of energy in the motor and the coupled mechanical system is reduced below a given value. This is an advantage particularly in connection with the braking of mechanical systems having a very low friction or inertia, since the system does not necessarily brake automatically upon the interruption of the power supply, but works mechanically toward a given end point. The invention thus enables such systems to be handled without problems, while ensuring that braking may take place under complete electric control.
The circuit is adapted to establish the interruption, the inversion or the correction after a certain delay by suitable control of the relays incorporated in the circuit, it is possible to perform switching to the unenergized contact point of the relays without this switching being superimposed by great powers generated by the motor during its braking.
In the given positions of the relays, the inductive load is connected to a power supply via at least one electrically controllable contact, preferably a FET transistor, which is interrupted by the deactivation of one or both relays after the activation of the relays, a particularly advantageous embodiment of the invention is achieved, since the supplementary interruption of the system from the power supply ensures that relay switching uncertainty does not result in unintentional connection to the power supply during the deactivation of the relays.
A typical combined control of relays and FET transistor or FET transistors will be implemented such that the FET electronics interrupts the connection to the power supply before relays, if any, are activated according to the invention.
Conversely, the connection of the FET electronics and thereby connection of the power supply will be established only when the relays have been set to the desired direction of rotation of a motor coupled to the circuit.
Several relay H-bridges may be connected in parallel for each FET transistor, just as the control may be performed with various control technologies, such as individual analogue control of each channel in a CB consisting of several channels, common analogue control of all channels in parallel or by means of a microprocessor control.
The relays of the circuit connect the motor to the negative terminal of the power supply in their non-activated state, a particularly advantageous use is achieved according to the invention, as the unenergized state thus does not require a power supply. The embodiment is thus particularly advantageous in connection with battery applications.
The system is coupled with an electric load instead of a decided inductive load, an increased service life of the relay-controlled systems is achieved according to the invention, if the loads generate voltage and current across the contacts of the relay even after the system has been cut off from the power supply.